One example of a known secondary-side power receiving circuit for contactless power feed equipment is disclosed in Japanese Patent Laid-Open No. 11-178104.
In the known secondary-side power receiving circuit for contactless power feed equipment, a pickup coil is opposed to a primary-side induction line that receives a high-frequency current at a frequency of, e.g., 10 kHz, the pickup coil receiving an induced electromotive force from the primary-side induction line. A resonant capacitor is connected in parallel with the pickup coil so as to form a resonant circuit resonating with the pickup coil at the frequency of the primary-side induction line. The resonant circuit is connected to a rectifying circuit (full-wave rectifying circuit) to feed power through a constant voltage control circuit to a load having varying power consumption (e.g., an inverter for controlling the traveling motor of a motor vehicle).
The constant voltage control circuit includes a choke coil, a diode, an output capacitor (voltage capacitor), a switching device (e.g., a transistor for power adjustment) that switches between a closed condition and an opened condition across the output ends of the rectifying circuit, and a control circuit that controls the switching device.
Under the control of the control circuit, an output voltage (the voltage of the load) is measured, the load is reduced by stopping the traveling motor, the output voltage (a voltage across the output capacitor) increases, and then the output voltage exceeds a preset reference voltage. At this point, the switching device is closed to stop power supply from the rectifying circuit to the load and interrupt charging of the output capacitor. Power is then supplied to the load from the output capacitor, and thus the output voltage is reduced to be less than the reference voltage. This opens the switching device to supply power from the rectifying circuit to the load and charge the output capacitor to reset the output voltage to the reference voltage.
The action of the configuration of the secondary-side power receiving circuit will be described below.
When a high-frequency current having a frequency of, e.g., 10 kHz is supplied to the primary-side induction line, magnetic fluxes generated on the primary-side induction line causes an induced electromotive force on the pickup coil, and then a current generated on the pickup coil by the induced electromotive force is rectified by the rectifying circuit. When the switching device is opened, the current is supplied to the load through the constant voltage control circuit. When the load decreases and the output voltage exceeds the preset reference voltage, the switching device is placed in a closed condition so as to supply power from the output capacitor to the load, thereby reducing the output voltage. When the output voltage is lower than the preset reference voltage, the switching device is placed in an opened condition to reset the output voltage to the reference voltage.
Unfortunately, the known contactless power feed equipment causes the following problems:
In the known secondary-side power receiving circuit for contactless power feed equipment, the output voltage is set at 0 V or close to 0 V in an initial condition for starting power supply to the primary-side induction line by switching control for monitoring the output voltage to be set at the reference voltage. Thus, the switch is continuously opened so as to require a 100% load (full-load condition).
Hence, a large number of devices or apparatuses in the full-load condition may be overloaded so as to exceed the rated power of a power supply device, activating the protective function of the power supply device to interrupt power supply to the induction line. This may prevent power supply to the devices or apparatuses. To avoid this problem, unfortunately, the number of devices or apparatuses to be operated needs to be considerably reduced at the start of power supply.